Two competing IMT-Advanced technologies are based on Long Term Evolution (LTE)-Advanced and IEEE 802.16m. 3GPP is developing an IMT-Advanced specification based on a newly completed LTE standard, which is to be adopted as the 4G standard. The air interface specification of Wireless Interoperability for Microwave Access (WiMAX) has been provided by the IEEE 802.16 standard, and the IEEE 802.16 Task Group m (TGm) is actively developing evolutionary specifications based on existing IEEE 802.16 and WiMAX profiles for IMT-Advanced submissions.
However, LTE and IEEE 802.16 have adopted different values of subcarrier spacing. LTE has fixed its subcarrier spacing to 15 KHz, but IEEE 802.16 has made its subcarrier spacing a variable for different sets of system bandwidths, namely 10.9375 KHz, 7.8125 KHz, 9.765625 KHz, etc. It is desired that the design of IEEE 802.16m enable it to be a cost-effective, global and competitive technology well into the future. Meeting these needs will require some balance between how IEEE 802.16m will be constrained by the requirement to support legacy mobile stations while continuing to meet the needs of a global IMT-Advanced technology.
IEEE 802.16m SRD requires that IEEE 802.16m shall meet the IMT-Advanced requirements. And all enhancements included as part of IEEE 802.16m should promote the concept of continued evolution, allowing IEEE 802.16 to maintain competitive performance as technology advances beyond 802.16m. On the other hand, the IEEE 802.16m standard requires that IEEE 802.16m shall provide continuing support and interoperability for a Wireless MAN-OFDMA Reference System which is defined as being system compliant with the capabilities set specified by the WiMAX Forum Mobile System Profile Release 1.0. For example, based on the backward compatibility requirements, 802.16m base stations shall support 802.16m and legacy mobile stations when both are operating on the same RF carrier.
However, there are problems existing in current legacy system designs. Some of them have an unfavorable impact of system implementation, network deployment and equipment cost. Thus, it is desirable to prevent the inheritance of the legacy systems' drawbacks as the 802.16m system is designed.